As soon as you connect your USB disk to your computer, the Operating System and any program that is running on it can have access to your USB disk data (unless it is blocked by Anti virus or something). And it is not hard for a malware to copy itself on your USB disk (infect it so to speak) or read and copy its data.

For example if it is a bitcoin specific malware it can read your USB, analyze the files to see if it can find a wallet file, a list of private keys, ... and using those private keys it can broadcast it to the attacker or sweep them!

If you want to transfer sensitive information like your private keys, then do it on a clean and "air tight gapped sorry, my English sucks sometimes!" computer. You can use a live linux without persistence which is disconnected from the internet.

1. Make sure your computer clean from virus/rat. scan it first by antivirus, antikeylogger, etc2. Turn off internet connection.3. Make folder with different name on your other USB disk.4. You can copy it safe now.

These things i would definitely do specially if those keys contained lots of bitcoins you will surely think up on secure transfer or copy of the key which n one could able to see non whats your doing. First of all having a clean computer free from virus would be really advisable and at the same time turn off internet since most remote viruses do really move on this way which do rely on internet connection to send out informations to its hacker.

I think its even you are using other OS its still not safe anytime soon if you are not using your usb for a long time or just few months usb can be damage i have lots of transcends usb disk i am using them for making bootable usb but i stop using it for few months but they are damage and never detect in computer or laptop. So if you really want to be safe the only thing that i know your bitcoin are safe is to buy trezor wallet you can use it offline and save your bitcoin for a long time. its immune to viruses and keyloggers..

If you are going to transfer keys there would be a possibility that it may copy or it can be used by somebody. And we all know that USB is just a universal port of sharing and receiving information such as photo and video. But most of it do not support further security and do not provide safety for you account. So if you want security at its finest you must need to find a more reliable tangible wallet. Such like Trevor that is offline wallet that promises security and anti virus for you safe keys.

it depends - what else on the value you want to transfer. This has been discussed many many times, here in the forum and reddit, and Trace Mayer in his bitcoin podcasts repeats it every time - search for it

Generally:for a couple of Euros/Pounds/Dollars (up to 100 maybe), it is ok to use a normal machine and wallet (or iPhone and Android). Similiar to your normal wallet for FIAT money. To then copy via USB, use a machine, that has been verified for malware. When you have values in the 1000 Euros/Pounds/Dollars range, you pay more attention, and copy this stuff from a secure, unixoide (xBSD or Linux LIVE CD) air gapped machine.At this point in time a cold storage wallet like Trezor or similiar has it's value.When you have more than 10.000, then you need to spend some time into re-thinking your store of value. Trezor is not secure enough, cause we cannot see, what is "below" the device. Here you would need to have your cold storage setup along with multisig. Do it yourself, make it trusted, and reliably tested. There is no final solution.Summary: low value, little effort in securing your keys, high values, much effort securing your keys. spend two dollars in security efforts for low bitcoin values, spend 1000 dollars for security when bitcoin values are above 10.000 Dollars, and yes, spend much more, when it's a million value. You are your own bank!